The present invention relates to videotape recording and more particularly to an apparatus to insure the proper phase synchronization between adjacent sequences.
Magnetic videotape recordings are frequently produced by the so-called diagonal track process. In accordance with this process, the magnetic tape is helically wound about a cylindrical drum while a video head or series of heads rotate in a gap perpendicular to the drum axis. As a result, diagonal tracks are recorded on the tape which are parallel to one another and extend diagonally across the tape. Each diagonal track may contain a video frame or field. The video heads rotate at a constant velocity and hence the angle of the diagonal track depends on the tape speed. Thus, to insure proper reproduction, it is essential that the tape speed be precisely maintained during both recording and playback.
To insure the proper maintenance of the tape speed, it has heretofore been proposed to provide a synchronizing track directly on the tape. The synchronizing track is generally recorded at an edge of the tape and is produced by a special fixed head independent of the image head during the recording process. During playback, the synchronizing track is sensed by the fixed head and a synchronizing signal is fed to a servo mechanism for controlling the speed of the tape transport to insure playback at the proper speed.
This prior art arrangement is shown in FIG. 1. In this figure, the magnetic tape 1 is transoorted by shaft 2 which in turn is driven by motor 3. A reference pulse source (not shown) is connected to terminal 4 which in turn is connected to a special transducer head 6 fixed with respect to the tape for recording the reference pulses onto tape 1 when switch 5 is in the "record" position. The frequency of the reference pulses is generally equal to the field frequency so that a one to one relationship exists between reference pulses and vertical synchronizing signals on the tape. The video recording head, which plays no part in the present invention is not shown in the drawings.
The reference pulses at terminal 4 are also connected to a servo mechanism 7 which controls the speed of motor 3. A tachometer-generator 8 is operatively coupled to the motor or shaft so as to respond to the speed of the tape transport by producing a train of pulses indicative of the speed of the motor. The tachometer pulses are fed through switch 9 to the servo mechanism 7 where the tachometer pulses are compared with the reference pulses and the motor is adjusted as required to minimize any error between the two.
When switches 5 and 9 are switched to their "playback" position, the mode of operation of transducer head 6 is changed to enable it to sense the pre-recorded pulses on a tape being played back. These pulses are amplified through amplifier 10 and fed to the servo control 7 through switch 9 where they are compared with the reference pulses so that the speed of the motor may be adjusted as required to insure playback at the recording speed.
As a result of mechanical tolerances in the tape 1 and head 6 some misalignment of the head and tape may occur between "recording" and "playback." Accordingly, phase shifting means 11 is provided to compensate for any such misalignment. Circuit 11 permits manual (or automatic) slight adjustment or shifting of the phase of the output of the transducer head to obtain the best signal-to-noise ratio of the output signal.
The above described system works adequately where uncorrelated sequences are recorded. However, synchronization problems arise if after a first sequence a second or more sequences have to be assembled and later reproduced without interruption. In this case the first reference pulse recorded with the additional sequence will be at a random location with respect to the last reference pulse on the first sequence. Thus, when the two sequences are played back together, a distortion or disturbance of the recorded image will occur at the splice point. Also, when the assembling point is reached in playback, the phase shifter has to be reset to compensate for track errors of the second sequence.
Heretofore, various schemes have been proposed to avoid this problem. One such prior art scheme is disclosed in German application DT-OS 2,339,406 which utilizes complicated and expensive components requiring counters, memory devices, auxiliary signal sources, comparison circuits and the like. In addition, calibration and monitoring of the different parts of the prior art circuit are critical and require a high degree of precision.
In view of the above, it is a principal object of the present invention to provide a simplified and relatively inexpensive system for recording and playing back video sequences in phase synchronization with one another.